In quantum optics, light-matter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger than the atomic dimensions [1, 2]. In contrast, here we experimentally demonstrate the vastly different giant atom regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit [3] to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal time-delay for the field to propagate across the giant atom, giving rise to non-Markovian dynamics [4]. We demonstrate the non-Markovian character of the giant atom in the frequency spectrum as well as nonexponential relaxation in the time domain. Following studies of the interaction between atoms and photons in cavity Quantum Electrodynamics (cavity QED) configurations, where the atom is placed inside a cavity to enhance the interaction strength with the radiation field [5,6], superconducting circuit quantum electrodynamics emerged in the last 15 years as an analogue to the cavity QED experiments. This has enabled probing the strong coupling regime of atom -light interaction to study exotic phenomena including vacuum Rabi splitting [7] and the controlled generation of non-classical photon states [8,9]. A later development was waveguide QED where superconducting circuits where coupled to open transmission lines [10,11,12]. These experiments replace natural atoms and optical cavities with Josephson junction-based superconducting circuits behaving 1 arXiv:1812.01302v1 [quant-ph]

No abstract

No abstract

Linear optical quantum computing provides a desirable approach to quantum computing, with only a short list of required computational elements. The similarity between photons and phonons points to the interesting potential for linear mechanical quantum computing using phonons in place of photons. Although single-phonon sources and detectors have been demonstrated, a phononic beam splitter element remains an outstanding requirement. Here we demonstrate such an element, using two superconducting qubits to fully characterize a beam splitter with single phonons. We further use the beam splitter to demonstrate two-phonon interference, a requirement for two-qubit gates in linear computing. This advances a new solid-state system for implementing linear quantum computing, further providing straightforward conversion between itinerant phonons and superconducting qubits.